[When should MRI for knee or hip osteoarthritis should be performed?].

Radiograph is the gold standard to establish the diagnosis of osteoarthritis (OA) and to classify patients in function structural severity according to Kellgren and Lawrence's classification. Radiograph should be performed on standing position for weight-bearing joints. In clinical practice, MRI is usually used to eliminate other diagnosis when X-rays are considered as normal and to precise abarticular structures and bone lesions affected in OA. This imaging technic allows to directly visualize articular cartilage damage with an excellent correlation compared to arthroscopy But MRI is also able to depict articular damages associated with OA such as bone marrow lesion (BML), osteophytes, cysts, joint effusion, synovitis, menisci lesions, tendinitis and bursitis. Some of them were associated with pain (BML, synovitis, effusion) while some articular lesions were more implicated in chondrolysis (focal cartilage lesion, BML, menisci lesion, synovitis effusion). In cases of X-ray abnormalities (osteophytes, joint space narrowing, bone condensation, cysts), menisci lesions should not be considered as responsible for pain in knee OA. Thus, MRI is the only imaging technic able to precise which articular structure is affected during the disease (bone, synovial tissue or abarticular tissues) and helps clinician to have a more targeted therapeutic approach.